Senior Mike Koogle, who was down the hall, said, "We thought it was someone slamming a door. Then people started yelling. A teacher ran in and said, 'lockdown.' "

"We are kids. We were terrified," Koogle said. He said they went into a classroom, turned out the lights and did their best to hide.

With the school on lockdown immediately after the shooting, some parents who had rushed to Perry Hall were unable to reach their children.

"Why aren't all the kids out? Why is my daughter not allowed out?" said Jennifer Short, who lives around the corner from the school.

She called the phone of her daughter, Taylor Trayband, a senior, and burst into tears when she answered. "Oh, my gosh, my baby." Still on the phone, Trayband eventually walked up Ebenezer Road, and Short ran to her and hugged her.

By early afternoon, some students were still being released from the school. One group quietly walked arm-in-arm toward Perry Hall Middle School.

Baltimore County Councilman David Marks, who lives next door to Perry Hall High School, said he received dozens of phone calls shortly after the shooting from worried parents and residents.

"It's horrifying," said Marks, a graduate of the school. "This is a very peaceful community, and my prayers are with anyone who's been impacted by this."

Former Baltimore County state Del. Alfred Redmer Jr. called the incident "tragic" and a reminder that "this can happen anywhere. We all need to be diligent."

But Redmer said the shooting doesn't mean there should be more gun control. "When disturbed people do unconscionable acts, the weapon that they use really is irrelevant, whether it's a stabbing or a gun or a baseball bat," he said.

One parent wondered how a student got a gun without his parents knowing about it.

"How does he have a gun? How do his parents not know he is this far gone?" said Lisa Eisenmann, who has two children at Perry Hall. "Somebody's baby was hit. Somebody's child is in Shock Trauma fighting for his life."

U.S. Education Secretary Arne Duncan was at the school last Wednesday to address 800 Baltimore County English teachers to boost morale.

"My thoughts are with the Perry Hall High School community — students, families and faculty — in this difficult time," Duncan said in a statement Monday. "Gun violence has no place anywhere, least of all in our nation's schools. I'd like to thank the local educators and law enforcement personnel who took action to help prevent additional students from getting hurt."

Baltimore County Police Chief Jim Johnson is scheduled to provide an update on the investigation Tuesday morning. Kamenetz and Dance are expected to attend.

While the suspect is a juvenile, anyone over 14 years old can be charged as an adult if the crime is one that, if committed by an adult, is punishable by life imprisonment or death.

While the shooter's motives are unknown, problems that generally move students to violence aren't always focused on until there is a tragedy, said Lisa Hurka Covington, founder and executive director of an advocacy group called SPEAK, or Suicide Prevention Education Awareness for Kids.

She said some schools, including Perry Hall, have offered programs in the past to help students cope with bullying, suicidal thoughts, acts of violence, depression and other issues that have led some to harm themselves and others.

"We're losing too many young people," said Covington, who lost a sister to suicide before founding her organization. "We need them to know there is help."

Baltimore Sun reporters Liz Bowie, Meredith Cohn, Ian Duncan, John Fritze, Mary Gail Hare and Alison Knezevich contributed to this article.